Conventional espresso coffee machines, used in dining places or at home, typically comprise a boiler assembly provided with a pump for generating pressurized hot water and an extraction assembly, by means of which the hot water is injected into an infusion chamber in order to extract a beverage from a substance in particulate form, typically ground roasted coffee.
The extraction chamber is formed through a filter holder, i.e., a body which is typically metallic or made of chromium-plated material and is cup-shaped in order to contain a filter, into which a dose of ground coffee is poured; the body is provided with a handle for engaging and disengaging the filter holder on the extraction assembly. The dose of ground coffee can be loose or enclosed in a tablet or pod composed of two layers of filter paper or filtering fabric which are connected to each other, for example a pod according to the E.S.E.® (Easy Serving Espresso) standard or a rigid cartridge or any other type of prepackaged dose suitable for preparing hot beverages.
The base of the filter holder has an opening in a substantially central position in order to allow the outflow of the extracted beverage. At this opening there is a spout with one or more outlets for conveying the extracted beverage into one or more cups.
The operations for engaging and disengaging the filter holder often entail friction between the internal surface of the filter-filter holder assembly and the internal elements of the extraction assembly, which cooperate with said internal surface in order to form an extraction or infusion chamber.
For example, a machine of a known type is provided with an extraction assembly which has a fixed O-Ring gasket, which acts radially on the internal surface of the side wall of the filter holder in order to ensure the hydraulic tightness of the extraction chamber. The movements for fitting and removing the filter holder with respect to the extraction assembly, which are performed every time the beverage is prepared, entail the friction of said internal surface against the gasket, causing its early wear or even its removal from its seat.
A particular known extraction assembly, fitted on machines known as “X1 by appointment” marketed by FrancisFrancis and comprising a gasket used as described above, provides, in addition to the traditional operations for fitting and removing the filter holder, the possibility to choose the mode of extraction of the beverage by varying the height of the extraction chamber by turning the filter holder about its own axis.
The bayonet coupling between the filter holder and the ring of the extraction assembly in fact allows to move the filter holder gradually closer to the hot water injector, varying the axial distance between the coffee pod and the hot water nozzle. The shorter this distance, the higher the hydraulic resistance of the cake of ground coffee, obtaining in output a so-called “ristretto” (strong) coffee. Conversely, a greater distance between the nozzle and the coffee pod reduces the hydraulic resistance of the pod, allowing to obtain a so-called “lungo” (weak) coffee in a time which is compatible with the normal use of the machine, avoiding the negative effect of “overextraction” of the beverage.
In this known extraction assembly, the fine adjustment of the rotation of the filter holder is important in order to be able to select the type of coffee to be extracted. Unfortunately, it has been found that the internal surface of the filter holder can slip on the gasket up to a maximum of approximately 20 mm of the length of said gasket before reaching the position suitable for preparing coffee.
This slippage, combined with the frequent rotations of the filter holder and with the high temperatures reached by the surface of the filter holder, entail rapid wear or breakage of the gasket and accordingly its frequent and inconvenient replacement.
Moreover, another drawback which is observed in espresso coffee machines is the friction between the filter holder and the gaskets or the internal walls of the extraction assembly with which the filter holder comes into contact. This friction often prevents easy removal of the filter holder from the extraction assembly at the end of beverage extraction, due also to small residues of beverage which may remain on the surfaces in contact.